1. Technical Field
The disclosure relates generally to integrated circuit (IC) chip fabrication, and more particularly, to methods of uniformly delayering an IC chip for electron holography analysis.
2. Background Art
During IC chip fabrication, chips are periodically delayered so that structures therein can be evaluated. One technique used to evaluate the structures is electron beam holography, which allows evaluations of junction profiles. Precision and uniform delayering is important for electron holography analysis of an IC chip. In order to obtain uniform delayering, specific methods have to be used. One challenge to uniform delayering, among others, are copper pads within the IC chip. These copper pads prevent material from being adequately polished during delayering, and hence, create non-uniform delayering. One approach to this problem is to repetitively use a chemical etching method and reactive ion etching (RIE) in combination with polishing for each layer. This approach, however, takes a relatively extensive amount of time, e.g., 2-3 days. Another approach is to use a colloidal polishing slurry to remove layers. However, this approach causes non-uniformity delayering problems on the copper pads, i.e., colloidal polishing slurry does not work very well on copper. If several copper layers are present, the end results of colloidal polishing are the copper pad of top metal layers remains while the surrounding area of metal is totally removed. If a structure near the copper pad needs to be examined, the colloidal delayering may result in a very small useable area, such as 10 to 100 nm scale. The requirements for electron holography, however, is about 10-20 μm scale. In addition, because the particle size of the colloidal is relative small (approximately 0.05 μm size), polishing by colloidal is also very time consuming.